body {
    background-color: #222;
}
div {
    background-color: orange;
    border-radius: 20px ;
    width: 300px;
    height: 100px;
    margin: 0 auto;     /*水平居中*/
    padding: 10px;
    position: relative;
}
.box:after {
    content: "";
    position: absolute;
    left: 50%;
    bottom: -15px;
    border-width: 15px 15px 0;  
    border-style: solid;        /*边框类型*/
    border-color: orange transparent;   /*透明*/
    width: 0;
}
